About 71 percent of the women called their flashes moderate to severe, enough to be disruptive. On average, they sweated it out for 8.8 years.
"I think expectation is a big part of women being upset about hot flashes," said study leader Ellen Freeman, a Penn women's health researcher. "If they're told it's going away in a year or two and it doesn't, that may make them more upset than if they're told what to really expect."
While hot flashes are a defining sign of menopause, they remain mysterious - and tricky to treat.
The most effective treatment is to replace some of the woman's ebbing estrogen. But hot flashes often resume when hormone supplementation ends. And extended use of hormone therapy has been discouraged since 2003, when a landmark study found that the risks of long-term therapy outweigh the benefits. Experts advise using hormone therapy for symptom relief as briefly as possible, not to exceed three to five years.
In theory, that should be plenty. The North American Menopause Society says "most women experience hot flashes for 6 months to 2 years." The American College of Obstetricians and Gynecologists says only a quarter of women "endure hot flashes for more than five years."
Many women would hotly dispute that statistic - and now, research supports them. A multinational survey of 8,000 Latin American women recently found that more than 60 percent were still flashing 12 years after menopause.
Freeman's study from last month in the journal Menopause is rigorous. It analyzed data from a subgroup of Philadelphia women in the Penn Ovarian Aging study. The subgroup were all premenopausal when they enrolled, gave blood samples, had no hormone therapy, and were interviewed annually for 16 years - a long follow-up.
They reported hot flashes before, as well as after, the final menstrual period, which is the official demarcation of menopause. While doctors may downplay it, flashes usually begin before the ovaries completely shut down.
More than a quarter of the women had moderate to severe flashes in the four years before menopause, and over a third had such sweats for five years after.
Like past studies, the new one found flashes were more common in women who were obese, African American or anxious.
What's a midlife female to do? Antidepressants, called SSRIs, are the only approved hot-flash care besides hormone supplements.
But Freeman believes a dose of realism can reassure women that they're not alone in fleeting furnacelike flushes - and that this, too, shall pass. Or at least ease up. "I think women want to know: How long am I going to have to deal with this?" she said. "Feeling misled doesn't help."